1. Field of the Invention
The present invention generally relates to a wireless bandwidth allocating method and a wireless base station. For example, a wireless bandwidth allocating method and a wireless base station used for a wireless communications system where user packets are encapsulated and communicated between a wireless base station and a wireless terminal.
2. Description of the Related Art
As an example of a wireless communication system that conducts wireless bandwidth allocation, there is a technology recommended for standardization by the IEEE 802.16 Working Group.
In the IEEE 802.16 Working Group, the technology is referred to as WiMAX (Worldwide Interoperability for Microwave Access) and is describes as Point-to-Multipoint (P-MP) type communication method enabling plural terminals to connect to a wireless base station. IEEE 802.16 describes two specifications of the technology which are IEEE 802.16d mainly used for fixed communications (see IEEE Std 802.16 (tm)-2004) and IEEE 802.16e mainly used for mobile communications (see IEEE Std 802.16e (tm)-2005). Although the specifications describe plural physical layers, OFDM (Orthogonal Frequency Division Multiplexing) or OFDMA (Orthogonal Frequency Division Multiple Access), for example, are mainly used.
FIG. 1 shows a service image of the IEEE 802.16d/e. As shown in FIG. 1, IEEE 802.16d/e is based on P-MP connection where plural mobile stations (MS) (wireless terminals) are connected to a single base station (BS).
In a case where a MS transmits data according to IEEE 802.16, the BS allocates a wireless bandwidth to the MS. In this case, in order for the MS to request allocation of a bandwidth by the BS, the MS transmits a Bandwidth Request CDMA Code (hereinafter referred to as “BR code”).
FIG. 2 is a diagram showing a sequence performed until a bandwidth used for transmitting data is actually allocated. In FIG. 2, when the BS receives the BR code from the MS, the BS transmits a UL-MAP message including CDMA_Allocation-IE for allocating a bandwidth enabling the MS to transmit a Bandwidth Request Header (hereinafter referred to as “BR header”) to the BS.
When the MS receives the UL-MAP message from the BS and a bandwidth enabling transmission of the BR header is allocated to the MS, the MS transmits the BR header to the BS. The BR header includes a Connection ID (CID) and the amount of data desired to be transmitted (number of bytes). The BS can obtain QoS (Quality of Service) data from the CID. The Qos data are exchanged between the BS and MS at the time of establishing their connection.
Then, the BS determines whether to allocate a data transmission bandwidth (bandwidth for conducting data transmission) requested by the MS with consideration of the obtained QoS data. That is, in a case where the BS receives requests from plural MSs, the BS allocates bandwidths by prioritizing connections requiring a high QoS. The allocation of the bandwidth is conducted with the UL-MAP message. Then, the MS, using the bandwidth allocated with the UL-MAP message, transmits data (MAC-PDU) to the BS.
FIG. 3 shows a format of a UL-MAP message including a CDMA_Allocation_IE. As shown in FIG. 3, a MS, which has transmitted a BR code, determines that a wireless resource has been allocated to the MS itself based on a Ranging Code, a Ranging Symbol, and a Ranging Subchannel included in the CDMA_Allocation_IE.
Meanwhile, since the BS is able to identify a CID requiring a wireless resource when receiving, for example, a BR header from a MS, the BS can allocate a wireless resource with a UL-MAP having a format different from the format of the UL-MAP generated in response to a received BR code.
FIG. 4 shows an example of a UL-MAP message generated in response to, for example, a BR header. By comparing FIG. 3 and FIG. 4, it can be understood that the data allocated in response to the BR header require fewer bits than the data allocated in response to the BR code. Therefore, the overhead of control data can be reduced for the data allocated in response to the BR header.
FIG. 5A shows a format of a BR header, and FIG. 5B shows the meaning of each field of the BR header. As shown in FIGS. 5A and 5B, the BR header is transmitted in units of CIDs and is able to express a request for bandwidth of approximately 524 KB. Furthermore, the type of the request of the BR header may be an incremental type or an aggregate type. The incremental type indicates the amount of data newly requested to be allocated. The aggregate type indicates the total amount requested. The aggregate type BR header is transmitted periodically.
In addition to the main message, the bandwidth request includes a piggyback request (incremental only) according to a Grant Management Subheader.
FIG. 6(A) shows a format of a Grant Management Subheader (PBR: Piggy Back Request) and FIG. 6(B) indicates the meaning of the Grant Management Subheader Field. As shown in the following FIG. 6(C), the PBR, which is added to the data to be transmitted by the MS (i.e. MAC-PDU (Packet Data Unit), is transmitted to the BS.
FIG. 6(C) shows a format of a MAC-PDU. The MAC-PDU has a GMH (Generic MAC Header) located at its prefix. A CRC (Cyclic Redundancy Code) used for detecting bit error is located at the end of the MAC-PDU. The MAC-PDU also has a SDU (Service Data Unit) including user data such as IP (Internet Protocol) packets. The PBR is transmitted between the GMH and the SDU.
The above-described BR header or the PBR is for notifying the BS the number of bytes equivalent to the MAC-PDU to be transmitted. The description “The request shall not include any PHY (physical layer) overhead” corresponding to the BR field in FIG. 5(B) means that incremented data are not accounted for. For example, although the amount of data to be transmitted is increased two times in a case where the encoding rate of the error correction code is 1/2, the incremented part of the increased data is not included in the BR. This is because the encoding rate changes according to the radiowave environment;
In order to efficiently use wireless resources, encapsulation can be performed on the SDU. For example, a packing process can be performed where plural SDUs are stored in a single PDU and transmitted or a fragmentation process can be performed where a single SDU is divided and transmitted as plural PDUs.
In a case of performing the packing process or the fragmentation process, a subheader including a sequence number is inserted in the PDU.
FIG. 7 is a schematic diagram for describing the packing process. In a case of combining (packing) plural MAC-SDUs into a single MAC-PDU, a Packing SubHeader (PSH) is used for adding a control bit indicative of the location of a fragment sequence number of a SDU and an SDU length to the MAC-PDU. SDUs having the same CID are packed into the same PDU. This can be understood since the GMH is shared by the plural SDUs.
There are three types of formats of the packing subheader (PSH) depending on factors such as whether there is an ARQ (Automatic Repeat ReQuest).
The types are an ARQ-enabled connection shown in FIG. 8(A), an ARQ-disabled and Extend-Type connection shown in FIG. 8(B), and an ARQ-Disabled and non-Extended-Type connection shown in FIG. 8(C). FIG. 8(D) is for describing each field of the PSH.
FIG. 9 is a schematic diagram for describing the fragmentation process. In a case of dividing a MAC-SDU into plural MAC-PDUs and transmitting the plural MAC-PDUs, a fragmentation subheader (FSH) is used for adding control bits indicative of a sequence number and the location of a fragment of a SDU and an SDU length to the MAC-PDU.
There are three types of formats of the fragmentation subheader (FSH) depending on factors such as whether there is an ARQ (Automatic Repeat ReQuest).
The types are an ARQ-enabled connection shown in FIG. 10(A), an ARQ-disabled and Extend-Type connection shown in FIG. 10(B), and an ARQ-Disabled and non-Extended-Type connection shown in FIG. 10(C). FIG. 10(D) is for describing each field of the FSH.
Although both the BSN (Block Sequence Number) and the FSN (Fragment Sequence Number) are sequence numbers, the FSN is incremented once with respect to each fragment of MAC-SDU whereas the BSN is not incremented once with respect to each fragment of MAC-SDU.
The MS performs encapsulation (packing (combining)) or fragmentation (dividing) according to wireless resources provided from the BS. Since a single SDU is divided into plural parts and transmitted in a case of performing fragmentation, overhead corresponding to, for example, a header or CRC may be generated.
FIG. 11 shows an exemplary process in which overhead is generated by the performing of fragmentation.
In FIG. 11, the MS requests the BS to allocate wireless resources amounting to a total of 1510 bytes (6 bytes for a header and 4 bytes for a CRC are added to 1500 bytes) for transmitting 1500 bytes of SDU (IP packet).
The BS allocates 500 bytes of wireless resources from its available wireless resources to the MS. At this stage, the BS recognizes that 1010 bytes remain to be allocated. Meanwhile, the MS, having been allocated the wireless resources, divides the SDU into a part of 488 bytes and another part of 1012 bytes. Then, the MS forms a PDU of 500 bytes by adding a header, a FSH, and a CRC to the SDU part of 488 bytes and transmits the PDU to the BS. In addition, the MS newly adds a header, a FSH, and a CRC to the remaining part of 1012 bytes, to thereby form a PDU of 1024 bytes.
At this stage, although the BS recognizes that 1010 bytes remain to be allocated, the amount of data existing in the MS is 1024 bytes. Thus, the MS reports this difference to the BS. In this example, after 1510 bytes worth of wireless resources are allocated by the BS, the MS sends an additional request for 38 bytes worth of wireless resources. Alternatively, an additional request may be made by using, for example, a piggyback request when a necessity for additional wireless resources arises.
Thus, in order to request wireless resources for the newly generated additional overhead, it becomes necessary to transmit a BR header or a piggyback request to the BS. This leads to a problem of wasting of wireless resources.
On the other hand, in a case where the MS combines plural SDUs into a single PDU and transmits the PDU (packing), the overhead can be reduced. In this case, since the BS is unable to recognize the amount of the reduction, the BS excessively allocates wireless resources to the MS. This also leads to the problem of wasting of wireless resources.